1. Field of the Invention
The present invention relates generally to the removal of particulate and vapor phase contaminants from a gas stream. More specifically, the present invention relates to the removal of particulates, such as fly ash and vapor phase contaminants, such as mercury, from a gas stream, such as a flue gas stream from a coal-fired boiler.
2. Description of Related Art
Utility power plants, in particular, coal-fired power plants, remove particulate matter, such as fly ash, from flue gas generated by the boiler before releasing the flue gas to the atmosphere. Typical methods for removing fly ash include the use of an electrostatic precipitator or baghouse filter. The collected fly ash must then be properly disposed of, taking into account its composition.
Utility power plants also are concerned about emission of trace metals in light of the 1990 Clean Air Act Amendment (CAAA) on air toxics (Title III). Special attention has been given to mercury (Hg) in terms of its environmental release and impacts, and the Environmental Protection Agency (EPA) is closely scrutinizing sources that emit mercury. Mercury is present in flue gas in very low concentrations (<1 ppb) and forms a number of volatile compounds that are difficult to remove. Specially designed and costly emissions-control systems are required to capture these trace amounts of volatile compounds effectively.
Several approaches have previously been adopted for removing mercury from gas streams. These techniques include passing the gas stream through a fixed or fluidized sorbent bed or structure or using a wet scrubbing system. The most common methods are often called “fixed bed” techniques. Approaches using fixed bed technologies normally pass the mercury containing gas through a bed consisting of sorbent particles or various structures such as honeycombs, screens, and fibers coated with sorbents. Common sorbents include activated carbon and noble metals such as gold and silver. In many cases where noble metals are used, only the surface layer of the sorbent structure is coated with the noble metal sorbent while the support underneath is made of ceramic or metallic materials. The sorbents in these fixed structures can be periodically regenerated by heating the structure and driving off the adsorbed mercury (see, for example, U.S. Pat. No. 5,409,522, entitled “Mercury Removal Apparatus and Method,” which is incorporated herein by reference in its entirety). The mercury driven off can be recovered or removed separately.
There are, however, several disadvantages of fixed bed systems. Gas streams such as those from power plant coal combustion systems contain a significant amount of fly ash that can plug the bed structures and, thus, the beds need to be removed frequently from operation for cleaning. In addition, fixed bed systems may produce a significant pressure drop in the gas path.
Alternatively, these beds may be located downstream of a separate particulate collector (see, for example, U.S. Pat. No. 5,409,522). Particulate removal devices ensure that components of the flue gas such as fly ash are removed before the gas passes over the mercury removal device. The beds will also have to be taken off-line periodically for regeneration, thereby necessitating a second bed to remain on-line while the first one is regenerating. These beds also require significant space and are very difficult to retrofit into existing systems such as into the ductwork of powerplants without major modifications.
In another technique, a primary particulate control device pre-collects most of the ash present in a gas stream. A sorbent is then injected into the gas stream downstream of the primary particulate control device but at a location upstream of a baghouse. A removable filter bag in the baghouse is then coated with the injected sorbent and contaminants are adsorbed as they pass through the filter bag (see, for example, U.S. Pat. No. 5,505,766, entitled “Method for Removing Pollutants from a Combustor Flue Gas and System for Same,” which is incorporated herein by reference in its entirety). In yet another technique, a porous tube of sorbent material is placed into the duct work through which the gas passes (see, for example, U.S. Pat. No. 5,948,143, entitled “Apparatus and Method for the Removal of Contaminants in Gases,” which is incorporated herein by reference in its entirety). Such a technique permits the tube of sorbent materials to be cleaned and the sorbent to be regenerated in place without having to stop the gas flow by heating the sorbent in situ and driving off the contaminants. However, application of heat to the porous tube while it is in the duct is not a convenient technique.
In yet another technique, a sorbent structure is coated with a renewable layer of sorbent, in which a flue gas passes over the sorbent structure (see, for example, published U.S. Pat. Application 20020124725 entitled “Method and Apparatus for Renewable Mercury Sorption,” which is incorporated herein by reference in its entirety). The sorbent structure can be a tube or plate and can be porous or non-porous and is placed inside a duct through which the flue gas flows.
In yet another process, a carbonaceous starting material is injected into a gas duct upstream of a particulate collection device. The carbonaceous starting material is activated in-situ and adsorbs contaminants. The activated material having the adsorbed contaminants is then collected in a particulate collection device. Such a process is described in U.S. Pat. Nos. 6,451,094 and 6,558,454, both entitled “Method for Removal of Vapor Phase Contaminants from a Gas Stream by In-Situ Activation of Carbon-Based Sorbents,” which are both incorporated herein by reference in their entireties. In this process, however, both particulate or fly ash and the sorbent having the adsorbed contaminant are collected together in the same particulate collection device.
While there are existing methods for removing fly ash from a flue gas stream and method for removing vapor phase contaminants, there remains a need for an improved method and apparatus that removes both fly ash and vapor phase contaminants from a gas stream and that allows for separate removal and collection of the fly ash and the vapor phase contaminant.